Collaborative Research: Surface water-groundwater connectivity in the Tidal Freshwater Zone and the fate of nitrogen in tidal rivers

人类通过食物和能源生产活动极大地改变了氮循环。增加景观的氮素负荷往往会导致氮素出口到海岸，导致藻类大量繁殖、死亡区和渔业衰退。去除河岸含水层和河床中的硝酸盐提供了一项重要的生态系统服务，减轻了海岸的氮素负荷。然而，绝大多数氮永远不会到达海岸，而是被保留在分水岭内或被微生物转化。河岸和河底是这种保留和转型的最佳地点，提供了宝贵的生态系统服务。在潮汐淡水区(TFZ)，潮汐将河水抽进抽出河岸和河床，去除硝酸盐可能特别有效。这项研究将确定TFZ的脱氮效率。如果自贸区在流域内的脱氮方面发挥了不成比例的大作用，管理战略应设法通过维持河岸缓冲区和限制可能堵塞河床的泥沙来源来保护自贸区，从而降低其清除效率。这项研究将提高我们管理淡水中硝酸盐的能力，并更好地评估潮汐淡水区和河口的生态系统服务。结果将通过两种方式与公众分享：(1)Creek Fest，这是一个向1000多名与会者推广流域管理的外展活动，PIs将与当地利益相关者讨论对TFZ管理的影响；(2)PIs还将开发一次虚拟实地考察，在内陆教室教育高中生和大学生潮汐环境中的生态系统服务。TFZ是河流和河口之间的动态过渡区，很少监测排放或营养物质通量。在自由贸易区，半日阶段的波动应该会促进密集的银行储存和释放。银行储存区可能是去除硝酸盐的有效场所。然而，相关的地下水位波动也可能使浅层地下水曝气，增强硝化作用，从而增加地下水中的硝酸盐。这两个潮汐诱导的相反过程的净影响可能从净硝酸盐产生到TFZ内的去除。该提案询问潮汐引起的水动力过程，如岸边储存和地下水位波动如何控制临海自贸区河岸含水层内的氮素转化，以及这些过程如何扩大规模以影响从陆地到海洋的氮通量。假设：1)TFZ水动力促进了河岸含水层中氮素转化的两个热点：土壤-地下水界面的硝化热点，半日水位波动使浅层地下水氧化，以及河流-地下水界面附近的反硝化热点，地表水通过耗氧沉积物交换；2)随着TFZ内下游潮差的增加，硝化产生的硝酸盐比反硝化去除的增加更多。这些假设将使用TFZ内的现场观测、实验室实验和数值模型相结合的方式进行验证。实地部分将确定河流-含水层界面上的流体和氮通量的特征，并确定克里斯蒂纳河流域(特拉华州)内临时开发区河岸含水层中的非保守氮素运移。沉积物岩心将用于实验室柱实验，以探索地下水位波动、地下水氧化还原条件和氮素转化之间的关系。数值模型将被用来提高克里斯蒂纳河流域TFZ沿线的氮素去向。

Humans have dramatically altered the nitrogen cycle through food and energy production activities. Increased nitrogen loading to landscapes often results in nitrogen export to the coast, leading to algal blooms, dead zones, and declines in fisheries. Nitrate removal in riparian aquifers and riverbeds provides an important ecosystem service that mitigates nitrogen loads to coasts. However a vast majority of nitrogen never reaches the coast, rather it is retained within the watershed or transformed by microbes. Riverbanks and bottoms are prime locations for this retention and transformation, providing a valuable ecosystem service. In tidal freshwater zones (TFZs) where tides pump river water in and out of the riverbanks and riverbed, nitrate removal may be particularly effective. This study will determine the nitrogen removal efficiency of TFZs. If TFZs play a disproportionately large role in nitrogen removal within watersheds, management strategies should seek to protect TFZs by maintaining riparian buffer zones and limiting sediment sources that could clog riverbeds, reducing their removal efficiency. The research should improve our ability to manage nitrate in freshwater and better value the ecosystems services of tidal freshwater zones and estuaries. The results will be shared with the public in two ways: (1) Creek Fest, an outreach event that promotes watershed stewardship to over 1000 attendees, the PIs will will discuss implications for TFZ management with local stakeholders; and (2) the PIs will also develop a virtual field trip to educate high school and college students in land-locked classrooms on ecosystem services in tidal environments. Rarely monitored for discharge or nutrient fluxes, TFZs are dynamic transition zones between rivers and estuaries. In TFZs, semidiurnal stage fluctuations should promote intense bank storage and release. Bank storage zones may be efficient sites of nitrate removal. However, associated water table fluctuations may also aerate shallow groundwater and enhance nitrification, adding nitrate to groundwater. The net effect of these two tidally induced, opposing processes could range from net nitrate production to removal within TFZs. This proposal asks how tidally induced hydrodynamic processes such as bank storage and water table fluctuations control nitrogen transformations within the riparian aquifers of TFZs and how these processes upscale to influence nitrogen fluxes from land to sea. It is hypothesized that: 1) TFZ hydrodynamics promote two hot spots of nitrogen transformation in the riparian aquifer: a nitrification hot spot at the soil-groundwater interface where semidiurnal water table fluctuations oxygenate the shallow groundwater, and a denitrification hot spot near the river-groundwater interface where surface water exchanges through oxygen depleted sediments; 2) as tidal range increases in the downstream direction within the TFZ, nitrate production via nitrification increases more than nitrate removal via denitrification. These hypotheses will be tested using a combination of field observations within a TFZ, laboratory experiments, and numerical models. The field component will characterize fluid and nitrogen fluxes across the river-aquifer interface and identify non-conservative nitrogen transport in the riparian aquifer of a TFZ within the Christina River Basin (Delaware). Sediment cores will be used in laboratory column experiments to explore relationships between water table fluctuations, groundwater redox conditions, and nitrogen transformation. Numerical models will be used to upscale nitrogen fate along the TFZ of the Christina River Basin.